Polyolefins stereoregular

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

Mechanism of stereoregulation on the basis of the data on polyolefin stereoregularity. The structure of a polymer chain is the recording of events proceeding in the insertion of olefin molecules into an active metal-carbon bond. To understand the stereochemistry of the propagation reaction, the data on the stereoregular structure of polymer chains are important. Recently, for this purpose, C-NMR spectroscopy has been extensively used... 

V. STRUCTURE OF POLYPROPYLENE AND OF THE HIGHER POLYOLEFINS STEREOREGULARITY, CONFORMATION AND CRYSTALLINITY... 

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

Inspired by the design of metallocence catalysts, there has been research into the use of C2-symmetric Ni a-diimine catalysts for the preparation of polyolefins with stereoregularity. Such catalysts were shown to afford higher degrees of isotacticity in polypropylenes as compared to the standard C2v-symmetric catalysts, which afford mostly syndiotactic polymer [106], Coates and coworkers have studied... 

Also in the 1980s, the discovery of homogeneous stereospecific catalysts for the polymerization of 1-alkenes has opened up new prospects for research on stereospecific polymerization and stereoregular polyolefins. Ewen and coworkers79 achieved this discovery on the basis of earlier research on metallocenes in combination with alkyl-Al-oxanes by Sinn and Kaminsky.10... 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

Although many stereoregular polymers have a helical conformation in the solid state (5,96], the conformation is lost in solution in most cases, except in the case of some polyolefins with optically active side groups [12], because the dynamics of the polymer chain are extremely fast in solution. Therefore, isotactic polystyrene [15,16] and polypropylene [17] prepared with an optically active catalyst do not show optical activity due to a helical conformation. However, a helical conformation can be maintained in solution for some polymers having a rigid main chain or bulky side groups that prevent mutation to random conformation, and the conformation may... 

Organoaluminum compounds are widely used in the production of polyolefines and stereoregular elastomers (as components of catalyst complex), as raw stock in the production of higher alcohols and carboxylic acids, as additives for reactive fuels, etc. 

It is quite easy to obtain dependencies of C and relevant rate constants on stereospecificity of centers, if the polyolefin is subjected to fractionation on the basis of its stereoregularity. 

Ziegler-Natta catalyst (ZNC) A catalyst involved in creating highly structured (stereoregular) polyolefin polymer chains. Used to mass-produce polyethylene and polypropylene. 

Modern polymerization catalysis, as we know it, was triggered by the development of metallocenes and the concomitant understanding of relationships between ligand structure and polymer properties. The manipulation of these useful relationships has led to a renaissance in tlie synthesis of polyolefin materials having new stereoregularities and, therefore, precise control of polymer rheology. 

Before the 1970s, Ziegler-Natta catalysts for a-olefin production were normally prepared from certain compounds of transition metals of Groups IV-VI of the periodic table (Ti, V, Cr, etc.) in combination with an organoraetallic alkyl or aryl (Table I). Practically all subhalides of transition metals have been claimed as catalysts in stereoregular polymerization. Only those elements with a first work function <4 eV and a first ionization potential <7 V yield sufficiently active halides, that is, titanium, vanadium, chromium, and zirconium (7, Only titanium chlorides have gained widespread acceptance in crystalline polyolefin production. 

A good reason to look at history is to select those parts we should like to repeat. One part of science history that both polymer chemists and businessmen would surely like to see emulated is the unprecedented, explosive burst of creativity, invention, and successful development that occurred in the 1950s and gave the world a new class of polymers (stereoregular), a family of new plastics (linear and stereoregular polyolefins), a family of new synthetic rubbers, including the first duplication of a natural high polymer... 

Because the stereoregular synthetic rubbers are the subject of a separate chapter, this discussion will be concerned only with the polyolefins. Although the newer and more exciting members of this family are stereoregular polyolefins, the first member (high-pressure polyethylene) is not a stereoregular polymer moreover, the first synthetic stereoregular polymer (polyvinyl isobutyl ether) is not even a polyolefin. 

Natta and his coworkers proceeded rapidly and brilliantly to synthesize other stereoregular polyolefins (from 1-butene, styrene,... 

The original, simplest polyolefins, polyethylene and polypropylene, continue to dominate the scene, even after two decades, to such an extent that no other polyolefin even appears on the production charts. Nevertheless, a great many (we may assume all) available olefins have been tested, and many have been found capable of being converted to stereoregular polymers. As was mentioned above, poly(l-butene) and poly(4-methy1-1-pentene) are being offered commercially and may be expected to achieve significant volume in the future. Isotactic and syndiotactic polystyrene are of much theoretical interest (26) but are not yet commercial products. 

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